incorrect context in method `login`; the context is `window` instead of the instance of `Login`

bad-context.js

import React from 'react';

class Login extends React.Component {
  render() {
    return (
      <div className='Login'>
        <input type='text' ref='username'></input><input type='password' ref='password'></input>
        <button onClick={this.login}>Please Login</button>
      </div>
    );
  }
 
  login() {
    var username = React.findDOMNode(this.refs.username).value.trim();
    var password = React.findDOMNode(this.refs.password).value.trim();
    console.log(username, password);
  }
}
 
export default Login;